As used herein, radiation means an electromagnetic or corpuscular radiation able to produce directly or indirectly ions, or to displace atoms during its passage through the atoms and the molecules of matter. The sources of radiation may be natural (cosmic radiation in space) or artificial, such as radiation of photons (X rays, Y rays), neutrons, protons, or heavy ions present in the nuclear installations, in scientific instruments or also in medical devices.
Unlike the electrical signals, the optical signals have the advantage not to interfere with the ionizing radiations, hence the interest for the optic-fiber devices in a radiative environment. However, it has been observed that exposing an optical fiber to radiations induces a degradation of the optical performances, in particular an increase of the attenuation per unit length of the fiber (called Radiation Induced Attenuation, RIA). Applications in space environment of the EDFA (used as lasers for communication between satellites) or of the ASE sources (for the optical fiber gyroscopes in satellites) or of the fiber lasers are today limited by radiation induced optical attenuation (RIA) phenomena, due in particular to the ionizing radiations present in space.
Many studies have been performed on various types of optical fibers to analyse the effects of the RIA, by determining the causes and searching for treatments allowing to improve the resistance of the optical fibers to radiations.
As used herein, silica optical fiber (or just optical fiber) means an optical fiber based on silica, wherein the silica optical fiber can also comprise other elements (germanium, phosphorus, aluminum, boron, fluorine . . . ) and/or air, and can be doped or non doped.
On the one hand, the radiation induced attenuation depends on the type of radiation, on the dose rate and on the total dose received by an optical fiber. On the other hand, the radiation induced attenuation strongly depends on the composition of the fiber and varies in particular according to whether it is a fiber with a core made of pure silica, phosphorus-doped silica, or a rare earth-doped optical fiber.
Moreover, it is known that the addition of gaseous hydrogen diluted in silica is advantageous for the resistance to radiations. Indeed, the presence of hydrogen allows to remove some colour centers (point defects due to vacancies, interstitial defects). Hydrogen limits the RIA phenomena and hence improves the performance of the fibers in a radiative environment, and in particular space environment.
A conventional method of hydrogenation consists in exposing a silica optical fiber, doped or not, to a pressure of gaseous hydrogen going up to about 300 bars, at a temperature of 80° C. during a relatively limited time, of the order of 48 hours for a silica fiber of 125 micrometers diameter. This method allows introducing gaseous hydrogen in the silica.
However, the presence of gaseous hydrogen dissolved in silica, and in particular at the core of the fiber, translates into the appearance of many bands of absorption at different wavelengths and of various intensities (cf. J. Stone, “Interactions of Hydrogen and Deuterium with Silica Optical Fibers: A Review”, Journal of Lightwave Technology, Vol. LT-5, no. 5, pp. 712-733, 1987). Although hydrogen is generally considered as transparent in the C-band (1530-1565 nm), it is observed in practice, in the C-band, an attenuation background that increases with the dilution of hydrogen.
The use of deuterium, instead and in place of hydrogen, offers the same benefits in terms of RIA, but has the additional advantage that it limits the losses induced by the gas present in the core in the C-band, which improves in proportion the performances of the optical fibers in the spectral band towards 1.5 micrometers.
However, the incorporated hydrogen or deuterium does not remain naturally in the silica and desorbs as a function of time over a variable duration from a few hours to a few days as a function of the ambient temperature.
A solution to limit the desorption of the gaseous hydrogen is to apply a carbon coating of a few hundreds of Angstrom in order to render an erbium-doped fiber tight to the diffusion of gas (Zotov, K. V, Likhachev, M. E., Tomashuk, A. L., Bubnov, M. M., Yashkov, M. V, Guryanov, A. N., & Klyamkin, S. N., “Radiation-resistant erbium-doped fiber for spacecraft applications”, 4-7, 2007). The thin carbon coatings are commonly used in industrial applications to render a fiber tight to the diffusion of gas from the outside and towards the inside of an optical fiber. In particular, a tight carbon coating is applied to protect the fibers used in the petrol field as a temperature and/or pressure sensor during drilling operations, which are subjected to conditions where the temperature reaches 250° C. and where the content of gaseous hydrogen may be high. The thin carbon coating is applied during the fiber drawing.
However, such a tight coating is gas-tight and, in practice, it is therefore very difficult to charge a carbon-coating fiber with gaseous hydrogen through this tight coating. To diffuse gaseous hydrogen through a tight carbon coating, even a very thin one (i.e. with a thickness lower than about 50 nm), very restricting operational conditions are required: the carbon coating fiber has to be maintained under a high pressure of hydrogen (5 to 110 MPa), at a high temperature (about 200° C.) and during a very long duration (about twenty days). This method allows to efficiently incorporate hydrogen inside the tight-coating fiber and hence to reduce the radiation induced attenuation (RIA).
Nevertheless, the tight-coating optical fibers having undergone such an hydrogenation treatment exhibit a very strong absorption over the whole spectrum and in particular between 1.4 and 1.8 μm. Indeed, at a high temperature, hydrogen forms permanent bonds of the O—H type that induce a strong optical attenuation in the C-band, with in particular a very strong attenuation band at 1380 nm and also at 1270 nm and 950 nm. Moreover, it is observed an increasing absorption as a function of the pressure of hydrogen in the spectral domain of 1050 to 1350 nm. In reality, the attempts of gaseous hydrogen insertion by this high temperature method through a tight coating end in the fixation of hydrogen in the form of O—H bonds. Finally, this method does not allow to measure the quantity of incorporated hydrogen. Now, for a too strong concentration of incorporated hydrogen, it is observed a drastic fall of the efficiency of a laser based on such an erbium-doped optical fiber. In these conditions, the optical-amplification efficiency of the erbium-doped fibers is noticeably reduced, which puts into perspective the improvement of the fiber hardening.